Sintered electric resistance heating element



SINTERED ELECTRIC RESISTANCE HEATING ELEMENT Richard A. Kielfer, Camillo Konopicky, and Friedrich Benesovsky, Reutte, Tirol, Austria, assignors to Schwarzkopf Development Corporation, a corporation of Maryland No Drawing. Application March 25, 1953 Serial No. 344,666

4 Claims. (Cl; 29-1825) This invention relates to materials which exhibit strength and oxidation resistance at high temperatures and are particularly suited for resistor heating elements that can be operated without protective atmosphere and have a surprisingly high service life also in oxidizing atmospheres.

According to the invention, materials consisting of molybdenum, silicon and, if desired, further additions, meet these requirements.

According to the invention, a heating element ofthe outlined requirements may consist, for instance, of molybdenum silicides; and particularly advantageous are compositions of the ternary system, molybdenum-silicon-aluminum. Accordingly, the heating element may be composed, for instance, of molybdenum silicides and molybdenum aluminides; or of molybdenum-silicon and molybdenum-aluminum alloys; or of a molybdenum, molybdenum-silicon or molybdenum-aluminum skeleton infiltrated with an aluminum-silicon alloy; or of molybdenum aluminide and silicon.

Heating elements, according to the invention, may be fabricated by any of the methods known to the art. It is thus possible to prepare heating elements in the form of tubes or loops by casting. It appears, however, preferable to prepare the elements by techniques of powder metallurgy; for instance, by extrusion followed by sintering and, if desired, under application of pressure during sintering.

It is also possible to prepare porous bodies of molybdenum or molybdenum-silicon or molybdenum-aluminum pre-alloys and to infiltrate these bodies with aluminum-silicon alloys.

The following examples will clarify the principle of the invention:

Molybdenum-silicide of the composition MoSi is prepared, either by melting or by hot-pressing, and pulverized. After addition of a plasticizer, the silicide is extruded to form rods or tubes. The plasticizer is volatilized by heating and the compacts are then annealed (sintered) in a high-frequency furnace for 1-2 hours. The sintered materials thus prepared have metallic conductivity, satisfactory strength and very high oxidation resistance.

A material on molybdenum silicide-molybdenum aluminide basis is prepared in the following manner: About 5% of molybdenum aluminide is added to molybdenum silicide. The mixture is compacted in the presence of a plasticizer. After volatilization of the plasticizer at red heat, the compact is presintered at 1100 C. and then subjected'to a final sintering process by direct passage of current through the compact in vacuum or in reducing or oxidizing atmosphere.

It is recommended to subject the heating element, prior to service, to a short-time treatment in an oxidizing at mosphere in order to form an adherent and gas-impervious protective layer. This is particularly recommended when the final sintering operation has been performed in a non-oxidizing atmosphere. After an insignificant volatilization of small amounts of molybdenum trioxide, this 2,831,242 Patented Apr. 22, 1958 V "ice ing from the aluminides as well as lower molybdenum oxides take part in the formation of the adherent protective layer.

The advantage of protective layers containing aluminum oxide is seen in the formation of compositions of mullite or sillimanite character. The following example refers to the production of heating elements having a protective layer of this type.

70 parts of molybdenum silicide and 30 parts of molybdenum aluminide of the approximate composition MoAl are mixed and hydrostatically compacted to the shape of tubes. The compacts are pre-heated, in carbon dioxide, at 1200 C., and finally sintered by direct passage of current at about 1600 C. (Throughout the specification and claims, all proportions are by weight, unless otherwise specifically stated.)

Prior to the final sintering operation, the ends of the tube-shaped heating elements are suitably inserted in terminal tubes of increased thickness; sintering then provides bonding between the terminals and the ends of the heatides which become conducting at high temperatures, such as zirconium oxide and thorium oxide, as well as with additions which are practically non-coductive at high temperatures, such as aluminum oxide, beryllium oxide and silicon oxide. The choice of the oxide to be added is dictated by the intended use of the elements.

The use of oxide additions is illustrated by the following example:

A mixture of parts of molybdenum silicides, 10 parts of molybdenum aluminide and 30 parts of zirconium oxide is filled in a graphite die of cylindrical cross-section, the die than being introduced into a hot press where the charge is heated to 1400 C. under application of a pressure of l50200 kg./cm. The die may be heated by a high-frequency field or by utilizing the graphite punches of the press for resistance heating. v

The scope of the invention is not limited to molybdenum alloys; The molybdenum may be partly replaced by other refractory metals such as tungsten, tantalum,

. niobium or chromium.

We claim:

1. In an electric heater device, an elongated heater resistance body formed by sintering at an elevated temperature particles of its constituents, which constituents consist essentially of 60 to 70 parts by weight of molybdenum and silicon in proportions present in molybdenum disilicide, and 10 to 30 parts by weight of molybdenum and aluminum in proportions present in molybdenum aluminide (MoAl said constituents containing also up to 30% by weight of a refractory oxide selected from the group consisting of zirconium oxide, thorium oxide, aluminum oxide, beryllium oxide, silicon oxide, and mixtures of said oxides.

2. In a heater device as claimed in claim 1, having a resistance body which consists essentially of about 70% of molybdenum and silicon in the proportions present in molybdenum disilicide and about 30% of aluminum and additional molybdenum in the proportions present in molybdenum aluminide (MoAl 3. Ina heater device as claimed in claim 1, having a resistance body which consists essentially of about 60% of molybdenum and silicon in the proportions found in Heany -2 Jan. 29, 1907 4 Linz July 21, Linz May 7, Stern Sept. 4, Goetzel et a1 Jan. 1, Briney Nov. 25, Beidler Jan. 12,

FOREIGN PATENTS Australia Aug. 10,

OTHER REFERENCES Mellor: Comprehensive Treatise on Inorganic and Theoretical Chemistry, vol. 11, page 523; pub. by Longmans, Green and Co., N. Y. 

1. IN AN ELECTRIC HEATER DEVICE, AN ELONGATED HEATER RESISTANCE BODY FORMED BY SINTERING AT AN ELEVATED TEMPERATURE PARTICLES OF ITS CONSTITUENTS, WHICH CONSTITUENTS CONSIST ESSENTIALLY OF 60 TO 70 PARTS BY WEIGHT OF MOLYBDENUM AND SILICON IN PROPORTIONS PRESENT IN MOLYBDENUM DISILICIDE, AND 10 TO 30 PARTS BY WEIGHT OF MOLYBEDENUM AND ALUMINUM IN PROPORTIONS PRESENT IN MOLYBEDENUM ALUMINIDE (MOAL5), SAID CONSTITUENTS CONTAINING ALSO UP TO 30% BY WEIGHT OF A REFRACTORY OXIDE SELECTED FROM THE GROUP CONSISTING OF ZIRCONIUM OXIDE, THORIUM OXIDE, ALUMINUM OXIDE, BERYLLIUM OXIDE, SILICON OXIDE, AND MIXTURES OF SAID OXIDES. 